The inflammatory bowel diseases (IBD) affect over 50 thousand US veterans. Thirty percent do not respond initially and an additional 50% eventually lose response to even our most effective therapies (anti-TNF agents). Thus, there is an unmet need for novel therapeutics. Indeed, an oral drug that interacts specifically with S1P receptor-1 (S1P1) (i.e. RPC1063/ozanimod) has shown significant promise in IBD trials, while another is already FDA-approved for multiple sclerosis (i.e. FTY720). However, a critical need remains to understand the understudied mechanism/s through which these drugs modulate chronic inflammatory diseases. Our long-term goal is to understand how to manipulate the S1P pathway for therapeutic purposes in IBD. Our immediate objective is to understand dendritic cell-based mechanisms of action of novel S1P1-selective agonists with proven efficacy in preclinical and clinical studies. Based on the expression of S1P1 on pro-regulatory dendritic cells (DC), we hypothesize that S1P1-selective agonists modulate DC function, and/or their migration, where they modulate T cell differentiation. This hypothesis stems from our preliminary data that demonstrates that chronic inflammation alters S1P synthesis and degradation (promoting DC retention) in IBD mouse models and human IBD and that S1P1-selective agonism degrades S1P1, promoting mobilization of DC to mesenteric lymph node and ameliorating inflammation in clinically-relevant IBD mouse models. Our innovative approach takes advantage of 1. chronic mouse models of IBD that recapitulate many of the characteristics of the human disease, 2. new drugs with distinct downstream actions and 3. cutting edge microscopy and mass cytometry techniques that allow us to directly visualize the effects of these drugs on a living animal and directly on the human intestine. Our inter-institutional studies bring together our extensive expertise in lymphocyte traffic/IBD models at SDVAMC with that of The Scripps Research Institute: S1P pharmacology, novel imaging techniques at the La Jolla Institute of Allergy and Immunology and access to clinical trial samples (Receptos). Our rationale is that understanding the mechanism of action of these novel anti-inflammatories will lead to optimized drug design and minimize the risks related to the pleiotropic effects of non-selective S1P receptor agonists (e.g. FTY720) on cellular processes. The proposed research is significant as this pathway is evolutionarily conserved in mice and humans. Therefore, our results might directly translate to human IBD, enabling us to elucidate the mechanism of action of S1P1-selective agents. Furthermore, we may uncover new targets for therapeutic intervention to be modulated pharmacologically with oral drugs, at reduced cost of production and administration, compared with current antibody-based biologic strategies.